The present invention relates to an electronic device, particularly a high-sensitivity or high-gain photometer for measuring the light emissions which are very often associated with biochemical or chemical reactions as well as with some physical phenomena.
As is known, the apparatus for detecting and measuring extremely faint light radiation, generally termed "photometer", is a highly sensitive instrument which essentially comprises an element holding the specimen to be examined, a sensor-amplifier for the signals emitted by said specimen and an electronic device for processing the signal output by said sensor-amplifier.
It is also known that a photomultiplier is often used as sensor-amplifier for low-intensity light signals; essentially, said photomultiplier is constituted by a set of electrodes, i.e. by a photocathode, a plurality of dynodes and an anode, all of which are contained in a glass vessel in which a hard vacuum, normally at a pressure of less than 10.sup.-6 torr, is formed. This kind of photomultiplier allows to achieve high amplifications which vary according to the number of dynodes and to the power supply voltage.
The most significant characteristic of a photometer is its sensitivity. In fact, the higher the sensitivity, the greater the ability of the instrument to measure very faint light emissions and thus to aid the experimenter in studying phenomena which have not yet been investigated from this point of view.
Sensitivity is generally defined according to the type of phenomenon being measured, but in any case it is ultimately dependent on the value of the lowest intensity of the phenomenon being studied which can be detected without uncertainties, i.e. with a sufficiently high signal-to-noise ratio.
Generally, the amplitude or intensity of the light signal measured by a photometer mainly depends on:
its light-gathering efficiency, which is defined as the ratio between the amount of light gathered by the sensor during a given time interval and the amount of light emitted by the specimen during the same time interval; PA1 its light conversion efficiency, defined as the ratio between the amplitude or intensity of the signal emitted by the sensor and the amount of light received by the sensor in the given time interval.
The noise of a detector instead generally depends on phenomena which occur within the detector and on the subsequent signal processing chain.
The sensitivity of a photometer therefore ultimately depends on the ratio between the product of the two coefficients which characterize the gathering and conversion efficiency and of the intrinsic noise of the detection system.
It should also be considered that, although the measurement chamber of a photometer is kept in absolute darkness, the sensor nonetheless emits a signal even in the absence of a specimen. This signal is measured before introducing any specimen and this measurement, known as "dark signal", is then subtracted from the measurement made in the presence of the specimen in order to determine the net signal which is due to said specimen.
The sensor-amplifier is therefore the essential element of a photometer, and its design conditions the light conversion efficiency.
Maximum light-gathering efficiency is generally obtained by designing the apparatus so that the specimen can be placed as close as possible to the sensor, so that the solid angle under which the sensor "sees" the specimen is the widest possible. A considerable improvement in gathering efficiency can be obtained by using reflecting surfaces which surround the specimen and reflect toward the sensor the largest possible part of the light emitted by the specimen in other directions.
In order to achieve high sensitivity, on the other hand, attempts are made to reduce the intrinsic noise of the sensor, which is essentially thermal. A method currently used entails cooling the sensor to temperatures below the lower bend of the sensor sensitivity curve supplied by the manufacturer, which plots "dark emission" against temperature.
The solution of cooling the sensor to reduce its noise, however, forces to thermally insulate the sensor from the specimen, which usually must be kept at a different temperature which can vary according to criteria selected by the experimenter according to the phenomenon being studied, the behavior whereof is generally affected by temperature. Thermal insulation is obtained by using gates with double- or triple-glazing, with vacuum between the glass plates, so as to limit heat exchange between the space occupied by the sensor and the space occupied by the specimen exclusively to the part due to irradiation. In any case, the sensor is necessarily moved further away from the specimen, and thus the improvement in sensitivity obtained by reducing the thermal noise entails a considerable reduction of the light-gathering efficiency due, on one hand, to the significant decrease in the solid angle under which the sensor sees the specimen and, on the other hand, to multiple reflections on the surfaces of the glass plates of the insulating gate.
Maximum light-gathering efficiency and high sensitivity obtained by reducing thermal noise through sensor cooling are thus mutually contrasting and, in the current state of the art, mutually exclusive design targets.